Using three-dimensional classical ensembles, we have investigated the internuclear distance dependence of nonsequential double ionization (NSDI) of H(2) molecules by an 800 nm, 1x10(14) W/cm(2) laser pulse. For the internuclear distance R ranging from 2 to 12 a.u., the NSDI of H(2) provides rich correlation patterns in the two-electron momentum distributions. These correlation patterns essentially reveal different microscopic dynamics in NSDI process. Moreover, our calculations show that R approximately 4 a.u. is the critical distance for double ionization yield of H(2). These results are qualitatively explained based on the classical barrier expression model and back analysis.